1. Field of the Disclosure
The present disclosure generally relates to an implant, and in particular relates to fabricating an orthopedic implant having a resilient core.
2. Background
In order to dynamize a rigid metal-based implant, a softer elastomeric-based component is often placed between two rigid metallic elements. Such constructs have been developed for example for posterior dynamic stabilization systems for the spine or for the new generation of total disc replacements based on damping discs. In this case, a soft elastomer is set between two parallel metallic plates and provides an axial damping with motion/rotation in all six degrees of freedom.
However, bonding a soft material like an elastomer to a hard one like a metal requires careful engineering of the interfaces.
For long term implantation, a good adhesion strength under complex constant mechanical stress states is needed, and, in addition, the bond must be chemically stable (hydrolysis, oxidation). Usually, the metal surface is prepared in a manner to improve its compatibility towards a polymer. Standard methods include chemical etching using agents like chromic acid or phosphoric acid to create an optimal roughness or application of an intermediate layer, called a primer. Silane-based compositions are known as suitable primers.
Diamond-like carbon (DLC) coatings are primarily used to reduce wear or to avoid cold welding between moving parts. DLC is a typically amorphous mixture of various carbon species (sp2, sp3) with more or less hydrogen in it and some short-range graphitic or tetrahedral ordering. Because of the growth mechanisms used for DLC synthesis, these coatings typically exhibit high compressive stresses in the range of several GPa. The adhesion of DLC on a metallic substrate is thus often limiting in its use for long term application but can be stabilized with the selection of adequate interlayers. An example is an interlayer of tantalum (Ta) for CoCrMo substrates. For titanium (Ti) alloys, more sophisticated interlayers are being developed based on TaN or TaC interlayers. The surface energy of DLC can be tuned (e.g., raised) with surface treatments like exposure to an oxygen plasma or to UV light; with such processes the surface becomes either hydrophobic or hydrophilic, thus possibly improving its affinity to another material.
Elastomers are thermoplastics that can be processed by injection molding or extrusion, where granules of the raw materials are melted and pressed into a mold at a temperature higher than the melting temperature of the elastomer. The melt binds to the metallic surface to some extent. Monomers can be injected into the mold instead of the fully reacted polymers as well, in which case the polymerization happens in the mold and monomers have an additional opportunity to bind to other species present. This process is known as Reaction Injection Molding (RIM).
There remains a need for an improved implant coating.